An Isaac Newton Institute Workshop

Quantum Information Theory: Present Status and Future Directions

Proposal for a loophole-free Bell test using homodyne detection

Abstract

In their seminal 1935 paper, Einstein, Podolsky, and Rosen advocated that if "local realism" is taken for granted, then quantum theory is an incomplete description of the physical world. The EPR argument gained a renewed attention in 1964 when John Bell derived his famous inequalities, which must be satisfied within the framework of any local realistic theory. The violation of Bell inequalities, predicted by quantum mechanics, has since then been observed in many experiments, thereby disproving the concept of local realism. So far, however, all these tests suffered from "loopholes" allowing a local realistic explanation of experimental observations by exploiting either the low detector efficiency or the time-like interval between the two detection events.

Here, I will report on an experimentally feasible optical setup that
potentially allows for a loophole-free Bell test by using highly efficient homodyne detectors. A non-gaussian entangled state of two light beams is generated from a two-mode squeezed vacuum by subtracting a single photon from each mode with the help of standard single-photon detectors.

A Bell violation exceeding 1% is achievable for a 6dB squeezed light source using single-photon detectors with an efficiency as low as 10%, provided that the homodyning efficiency lies around 95%. Given the recent demonstration of photon subtraction from pulsed single-mode squeezed states, we envision that this proposal may lead to the first complete test of Bell violation in a foreseeable future.